I have 20 servers hosted on orderbox DNS hosting,I have set up opendkim on all servers with signing by domain say "example.com" i.e d=example.com in DKIM header of all the server outbound mails.
The public key generated by all the servers is different,

but i want the public key to be same in all the servers so that i get dkim="pass" by entering the universal public key in the DNS hosting TXT record entry.

Now save your private key to the dkim directory, where you configured Postfix or OpenDKIM to look it up. Distribute this private key on ALL 20 servers, and they will sign the mail the same way. And make the respective DNS TXT entries for the "public".

i followed this tutorial stevejenkins.com/blog/2010/09/… for configuring opendkim in postfix i have generated private and public keys on 1 server.Now are you saying to use this public key and private key on all servers after configuring opendkim in them i.e using public key in TXT recoeds and saving private and public key on say "cat /etc/opendkim/keys/example.com/default" and "cat /etc/opendkim/keys/example.com/default.txt" respectively?
–
achal tomarAug 20 '12 at 7:25

The private is the one which signes the emails, so you ned just that. So just copy this file /etc/opendkim/keys/example.com/default (path taken from the tutorial) and copy this file to all servers. Set upt DNS TXT the right way (test it once on the working server) and you are good to go
–
leviathanusAug 20 '12 at 18:09